This commit improves the listing of encrypted objects:
- Use `etag.Format` and `etag.Decrypt`
- Detect SSE-S3 single-part objects in a single iteration
- Fix batch size to `250`
- Pass request context to `DecryptAll` to not waste resources
when a client cancels the operation.
Signed-off-by: Andreas Auernhammer <hi@aead.dev>